add CI test to verify that installed packages (and versions) are the same for x86_64/aarch64/ppc64le

[boegel]

The recent Prefix bootstrap script added a "stable-only packages" mode (see https://gitweb.gentoo.org/repo/proj/prefix.git/commit/scripts/bootstrap-prefix.sh?id=92e83b4212ec23c1ea9d6013343462b85011a6bd), but only for amd64 (x86_64).

This could easily lead to different package versions being installed for x86_64 vs aarch64 and ppc64le.

That's already the case though, depending on when the compat layer is built.

So, we should check in CI that the same set of packages (and package versions) is installed in each of the compat/linux subdirectories.

[bedroge]

This CI can generate a list of the installed packages by running something like the following on all supported architectures (using qemu):

cd /cvmfs/pilot.eessi-hpc.org/2021.12/compat/linux/$(uname -m)/var/db/pkg && ls -d */* | sort

or

qlist -ICv

Then it can do a diff of the outputs, and use our yaml set definition (https://github.com/EESSI/gentoo-overlay/blob/main/scripts/eessi_sets.yml) to ignore certain architecture-specific packages.